Can I use an FTSs coil as a HERMS coil?

A HERMS coil needs more than double the surface area to be effective. To do it right, you would need roughly 50 feet of 1/2" tubing. That would ensure that your ramp speeds are strong and that the coil is effective at controlling temps.

Pumps move wort through coils quickly, so you want a longer coil with more surface area to maximize your performance.